1. Field of the Invention
The present invention is concerned with a liquid level control system, more particularly with a liquid level control system utilizable to regulate the flow of water through a hydraulic turbine in response to sensed water levels upstream and downstream of a flow restriction in the water course.
2. Description of Related Art and Conventional Practices
U.S. Pat. No. 2,092,623 (Kuster) discloses a pair of gas bubblers, one positioned upstream and one downstream of a trash screen through which a liquid flows. Both bubblers are connected via a pressure drop orifice to opposite sides of a mercury switch which controls a power rake used to clean the trash screen. The mouths of the bubblers are positioned at the same elevation so that a back-pressure differential, which develops between the bubblers when the downstream liquid level drops due to accumulated trash on the screen, displaces mercury to close the switch and actuate automatic raking of the screen. Clearing the screen tends to equalize the upstream and downstream levels, thereby reducing the back-pressure differential and opening the switch.
U.S. Pat. No. 3,476,538 (Trethewey) discloses controlling the flow of molten glass through a flow restriction by utilizing (FIGS. 1 and 3-5) a pair of bubblers, one positioned upstream and one downstream of the flow restriction and having their mouths at the same elevation. The FIG. 2 embodiment utilizes surface level detectors such as contact or pneumatic probes. The sensed difference in upstream and downstream levels is used to record and/or control the flow rate.
U.S. Pat. No. 3,573,019 (Rees) is also concerned with a flow measuring device for molten glass which uses a pair of bubblers which have their respective outlet ends facing, respectively, towards and away from the direction of flow of the molten glass (FIG. 1). The mouths of the opposite facing bubblers are at the same elevation and the pressure differential due to flow of the glass into one of the mouths and away from the other is used to monitor the flow rate.
U.S. Pat. No. 1,975,710 (Borden) shows a device for remotely recording the level of a body of liquid. A compressed-air bubbler is connected to a tank containing an indicating liquid and the feed of compressed air to the bubbler is made responsive to the surface level of the body of liquid by a float thereon which is connected so as to operate the compressed air supply valve.
U.S. Pat. No. 1,662,248 (Jacob) shows a weir flow rate gauge which includes a pair of leads positioned, respectively, upstream and downstream of the weir with the mouths or open ends of the leads being submerged beneath the liquid. The elevation of the mouths of the leads beneath their respective levels of liquid is immaterial.
The hydroelectric energy obtainable from flowing water is proportional to the product of available head times flow. Accordingly, energy generation is maximized if the rate of water flow through the hydroelectric turbine or turbines is commensurate with the highest available water level, i.e., the spillway crest of a dam. From this point of view, the most desirable location for measuring the available water level is adjacent to the spillway, which is upstream of the trash racks or trash screens used to protect the inlet to hydroelectric turbines. This location presents no problem if the water levels upstream and downstream of the trash screen are the same, or nearly so. However, if debris or ice obstructs the trash screen, the water level on the downstream side of the trash screen will drop and that on the upstream side of the trash screen will rise. A conventional water control system measuring only the upstream level will increase the turbine discharge, i.e., the flow rate through the turbine, in response to the rising upstream water level. The turbine thus operates in response to a false water head reading and the increased draw of water from the water box on the downstream side of the trash screen tends to deplete the water available in the water box and, if the screen is sufficiently clogged, the result may be consumption of nearly all the water downstream of the trash screen so that air as well as water may enter the turbines. Under such conditions, the turbines may eventually end up pumping water downstream and consuming rather than producing electrical power, although controls are usually provided for automatic shutdown of the plant as soon as such reverse flow of electrical energy is detected. Another, although less severe, disadvantage of such drawdown of water on the downstream side of a clogged trash screen is that the resulting high differential pressure across the debris on the trash screen may make removal of the debris difficult or impossible. Conventional practice for plants which utilize water control systems measured upstream of the trash rack is to switch the plant to manual control and operate at reduced output during periods of severe clogging of the trash screen.
On the other hand, if a plant utilizes a control system which measures the water level downstream of the trash screens, such plants operate at a somewhat reduced head at low flow rates even when there is only a small difference in between the upstream and downstream water levels, e.g., between the spillway and the plant intake levels. Thus, even minor clogging or blockage of the trash screen adversely affects energy output. These and other shortcomings of conventional control systems are overcome by the present invention.